OM and ON must be vertical
According to the known conditions, if the edge OM of BO is folded to coincide with OE, then ∠BOM=∠EOM.
In the same way, ∠CON=∠EON
∠∠BOM+∠EOM+∠CON+∠EON = 180
∴2∠EOM+2∠EON= 180
That is, ∠ eom+∠ eon = 90.
So the crease OM⊥ON
I hope I can help you, please ask questions if you don't understand!